U.S. Pat. No. 6,783,326 (Aug. 31, 2004), which the instant application incorporates by reference, broadly discusses uses of a panel of granular material: for filtration of a dusty gas, assisted by an accumulation of deposits of dust (filter cakes) upon gas-entry faces of the panel; for countercurrent contacting of the granular material with a gas.
The referenced patent discloses means for renewing gas-entry faces of a panel of a granular material in which faces are transversely disposed, upwardly spaced, and held in place by supporting members. One means is to provide a surge flow of gas toward the faces (a “puffback”), which produces a body movement of the granular material toward the faces, thereby producing spills of moieties of the material from the faces and concomitantly exposing previously underlying material, thereby renewing the faces. Another means is to strike the panel with the blow of a hammer, the blow being directed either upward or downward. The blow also gives rise to a body movement of granular material toward gas-entry faces, producing spills of moieties of material therefrom.
These means are admirably effective in renewing gas-entry faces when a supporting member is a louver seen in vertical cross-section to comprise two segments: an outer segment sloping upward from its outer edge (e.g., sloping at an angle of 9° to horizontal) and an inner segment sloping upward toward the louver's inner edge at, preferably, an angle not less than ˜45° to horizontal (where “outer” refers to the outside limit of the gas-entry side of the granular-bed panel, and “inner” refers to a location within the interior of the panel).
A disappointment, however, has been performance of these means when tested for renewal of gas-entry faces resting upon transversely disposed upwardly spaced flat-plate louvers. Following a puffback of flat-plate louvers (inclined upward from their outer edges at 9°), a narrow, sand-free region appears beneath each louver, extending to its upper, inner edge. A similar, sand-free region appears after subjecting flat-plate louvers to a sharp upward or downward motion; this region is even wider than the narrow sand-free region seen following puffback.
The appearance of this region is undesirable for two reasons. First, following gas-entry face renewal, granular material moves downward in a space between inner edges of support louvers and a panel's gas-exit side, the downward motion making good losses of granular material from the panel's gas-entry faces. If the panel is tall, at flat-plate louvers near its top, the downward motion pulls granular material away from part of gas-entry faces close by their inner edges. If the panel is employed for filtering dust from a gas, such pulling away of moieties of gas-entry-face material hurts filtration efficiency. Second, in an application requiring countercurrent contacting (i.e., an application requiring relatively wide flat-plate louvers), extension of a sand-free region to a louver's inner edge spoils the desired countercurrenticity of contacting in the granular material lying beneath this region.
Yet flat-plate louvers have important advantages over louvers comprising outer and inner segments, the latter sloping upward at 45°: flat-plate louvers afford significantly lower pressure drops and lower costs for fabrication.